I don't think they are underwatered, it looks like a kind fungus, try to put some good soil in the pot, spray with some garlic water and try not to take the pots out for a while, check your outdoor plants if they are affected!

They look fine to me, except the one spot that appears to be sunburned (a closer picture would help) I believe garlic water is used for pests and I cant see any signs of those. Snake plants are tough, yours look good, mine do not like direct sun at all.

Looks like sunburn to me. If they were inside and receiving not much light, a rather sudden increase, even if the lighting is indirect can cause sunburn. If the soil was on on dry side, it would make them even more prone to sun scald. Snake plants are one of the toughest indoor plants to be found. The damaged areas very unlikely to be fungal damage. You need a lots of humidity and little air movement for fungal problems to start. Snake plants have a very fibrous leaves with a heavy waxy coating that would make it hard for a fungal problem to take hold. The damaged areas may just turn dry or the the whole leaf may slowly fade away. As an ex professional plantscaper, when faced with a problem like this, the whole leaf would be removed with the cut being made at the soil line. Hope this helps.

I suspect wet soil too (but close-up photo would help a lot!).

2 weeks without water is not a problem for them. And they will do well indoors, without full sun exposure.

Leaf in the 1st photo looks sunburned - you said:..."take them out in the balcony under shade just for some spring boosting and just for 3 days"... Even in shade, the light outdoors is more intense than inside.

But I still would make sure drainage is good and the soil is not staying wet for too long. Is there drainage hole(s) in that container?