Flies in cage?

So I've had my boa a couple months now and have had no problems with her, recently however I've noticed small flies in her tub, she's in an 84 litre box with lots of holes for ventilation as the humidity was too high without them. I keep her on orchid bark and I was wondering if its possible the substrate had them in when I bought it, I'm not certain this is the cause as I had a similar problem with my Giant African Land Snails and they were kept on compost. I thought they may be getting into the tub but its winter now and there are no flies about and I never find any flies in my corn's tub which is right next to the boa's and also has lots of holes for ventilation. When I first noticed the flies I cleaned the tub and put in fresh substrate but within a few days they were back. I was wondering if anyone has had a similar problem and if they did how they dealt with it. It doesn't seem to be affecting the boa at all as she is still eating, shedding, etc fine and doesn't seem stressed at all but they're just annoying me to be honest and I want to get rid of them. Thanks in advance for any help or advice :)

If they are extremely small, they could be fungus gnats. If they are fruit fly sized, there may be some hidden poop in her tub that you missed that they are breeding on.

If you don't want to change substrate to aspen, etc. you can make a fly trap outside her tub. I put a little water, dish soap, coke or other sweet drink, and maybe a little vinegar. This works well with fruit flies, I am not sure about fungus gnats but it's worth a try.

I wouldn't mind switching to aspen as it would mean I only have to buy one type of substrate but I thought since it tends to mold when wet it would be a poor choice for a boa because I need to mist occasionally. I googled both fruit flies and fungus gnats and the flies in the cage are smaller than fruit flies but larger than fungus gnats. I just double checked and there is definitely no poop or urates that they could be feeding off. If aspen is suitable for boas I'll switch and hopefully it'll get rid of the flies and it'll mean I only have to buy one type of substrate. Could some other people give their opinions on switching to aspen and whether its suitable for boas, I just want to make sure I'm doing the right thing for her. Thanks for the fly trap suggestion Aldebono I'll try and get some sticky paper when I go to the shops next, until I can treat the cause of the problem this will definitely alleviate the symptoms. :)

The trap drowns them. The dish soap breaks the surface tension of the water and they fall in instead of walk on top. Thought I would elaborate.

We use aspen on our Hog Island boa just fine and she has whatever ambient room humidity there is. We are in Florida though so whole different atmosphere.

I get it so I fill a pot or jar with that solution and they get attracted to it then fall in and die, awesome. Yeah I went to Florida over Christmas and it was still hotter and more humid than it is in England in summer, best place I've ever been. Thanks for the advice hopefully if they are coming from my house that'll get them. :)