Looks like moisture or something liquid like getting on the drywall from above. If your question is what might they be.

It's in the kitchen? Could be condensation from boiling something on the stove without using the vent?

Seems like it didn't save my original description, this is what I wrote:

"Hi everyone,

new here. I moved in a new house in December, yesterday we noticed a few small spots in most of the rooms at first floor. On the ceiling where above we have the attic.

We are not sure if the spots were already there when we moved.

We know that some of the rooms had ventilation issues in the past, there was mold forming on the corners of external walls, due to improper/lack of ventilation, now it's solved as we are keeping heating and window vents always open.

Not sure if these were there already and caused by that old ventilation issue.

Any idea what could it be?

The spots are irregular and random, I checked the attic and they don't look to be in proximity of water tank, not sure if there are pipes below insulation though.

We were thinking to remove them, and keep an eye if they come back again, would you recommend this approach?

thanks for any suggestion"

I think to get a good answer, first you need to know, if it is coming from a roof leak, etc. You should be able to tell from a good look in the attic. Also move some of the insulation from the areas in question, to see, if there is a problem in that area. You can always put the insulation right back where you moved it from. It is much better to find the source, than to treat the symptoms.

I suspect bleed through from stains painted over just before you purchased the house. I would seal the spots by spot priming each with BIN shellac based primer. Then do another coat of the primer over the entire section containing the strains. Don't substitute cheap latex primer! Regardless of what some big box store employee tells you, there is a reason that shellac primer sells well despite it costing nearly double the cheap stuff.

thanks all for the comments. Have to do some works in the attic soon, so I will make sure to check below insulation to see if any leak visible. If not, I will probably take that they are old stains bleeding over through, and just prime/re paint the ceilings. We were planning that anyway for the summer, I just want to be sure there are no major leaks or issues before it's too late

I'm quite sure the previous owners weren't smokers, house was built in 2006. Is there anything else that could have caused these stains, beside nicotine? There is a fireplace, but it has integrated stove since a few years, could it be that when there was open fire that air caused these stains? I'm just curious to know why these appear as random spots instead of well defined areas like when there is some mold.

These are water spots on my ceiling. About 2 feet to the left is the fireplace. The chimney on the outside was in bad shape. We could see in the attic that water had gotten in and ran down the rafters and dripped to make these spots! We had a new roof put on and the chimney repaired. No more water getting in and as a bonus no more mice and squirrels getting in!

Do a thorough inspection in your attic.

My thought too - that you have a leaky roof or chimney, leaking sealant around a vent, or even poorly installed, clogged gutters that are directing water under the roof shingles during a heavy rain. I have had water spots show up on a ceiling several feet away from the actual site of water entry.

Since the spots are in the kitchen, I agree w/ other posters who have said they are likely bleed through from old stains.

BIN in the red can--'pigmented shellac'. If the primer isn't shellac, real shellac, don't buy it. Get the real deal, genuine old school shellac. Not the new 'synthetic shellac' versions. Buy a small can or a spray can & treat the stains. I always do two coats if I brush on. Maybe not necessary but it's easier to take a wee bit of extra time & precaution than to have to go back & reprime & repaint if one coat doesn't do.

If you use the spray can, then do three coats.

thanks all. Spots are on 1st floor and attic is above. 2 weeks ago we got the gutters guy coming and unblock them, they were badly clogged. I hope this might be the reason, but I'll follow the advice and have a thorough check in the attic for visible signs.

Sounds like the gutters would be a likely cause. You could cover the stains with the shellac primer now and that would give you a good opportunity to make sure they don't come back before your full paint job in a few months. Instead of darker spots to look at, you could have a few white spots instead

thank you all, very useful tips! Btw is this the guy you are recommending?

No, thats good primer, but, not what you are looking for. This is the one you are looking for.

I have similar spots in my kids bathroom and found it stems from a roof leak. Until I fix the roof, I'm on hold to doing anything in the bathroom, including the moisture issue I currently have. .I will invest in this BIN product once the roof is fixed

Seems like I can only find the primer-sealer stain killer, the ultimate stain blocker seems impossible to find. I sent an email to Zinsser, I am afraid that maybe it's a discontinued product? I'll let you know once they come back to me

You can buy a can of plain shellac and put on the spots, and then use regular primer over that, once its dry. Its the shellac, that seals the spots so they do not bleed through, and yes, this is available. I just bought a can of it last week.

this is the reply I got from supplier:

"The product you are looking at is the American version of the B-I-N the UK version is the red can https://www.zinsseruk.com/product/b-i-n/ you can get the B-I-N primer sealer in most decorating centres in Northern Ireland if in southern Ireland"

So they are essentially saying it is the same product.. I'm a bit confused?

They are saying that is their UK comparable product, so yes essentially the same product from their prospective. They are both a shellac based primer/spot sealer per the labels.

so in my case I'd say it's the best I can get here!