Insect frass would land in the tray only if plant foliage overhung an open area of the tray.

Was it by the edges of the tray or under the holes of the pot?

tj

if under.. i would tip the plant out of the pot.. and see if there is anything living in the bottom of the pot ...

was the plant outdoors for summer???

ken

There was a large amount under the pot where there are also holes, however the foliage does not hang over the tray at all. Two plants were outside at times during the summer but we would bring them inside after a few hours. I've examined the plants and I didn't notice anything out of the ordinary. The third plant I've noticed with the same issue has never been outside and is a succulent.

ken's advice is good.

I decided to see if there was in fact a stowaway in the dirt and I could not find anything crawling around so, I placed her back in the cactus mix to see if there would still be stuff under the planter. Turns out there was so I ditched that medium and potted her in some new soil. But I am still confused as this plant would have never been outside at all and I could not find anything in the soil. There is still another plant that has this same issue and that one was outside.