I was going to say, look for evidence of absence. Places where plants don't grow, the wind doesn't blow, and animals don't go. But then it hit me…
Look for dead birds. Invisible things (such as plate glass windows) and birds are natural enemies. Birds fly into invisible things. Birds die when they fly into invisible things. Dead birds fall to the ground and pile up unless disposed of in some manner. So along with dead birds, look for unusually high scavenger activity. Any place where swine, dogs, crows and ravens etc. congregate in unusual numbers. And are well fed for the local conditions. If the tower staff disposes of the avian corpses, look for signs of that. Routine chores rarely hold the attention of the worker the way special projects do, and so mistakes are made and things overlooked that would not be overlooked otherwise.
In the same vein, look for places birds fly around. Places where there is nothing visible, and yet the birds still detour.
In a case like this a Speak with Animals is a wonderful tool. But you need a Druid, so if you don't have one…
Anyway, I wish you and the rest of the party luck.