I am currently in a smoke-free hotel. I usually prefer to light incenses in the bathroom, which is located right next to the main door to the hall. I have put a towel under the main door in the past, and opened the bedroom window. Would burning incense in a smoke-free hotel be a problem? I had no people complain so far, but would people complain?

Yes, it could be a problem. You could be subject to a substantial fine.

It's a huge problem!

Smoke free means just that. And yes, it does apply to incense! The next guest in the room may have allergies and when they check in they're going to complain to the front desk. Then the front desk has to reconfigure the guest rooms and move the guest to another room and put that room you were in out of service.

well "smoke-free" refers to no smoking cigarettes pipes cigars and the like... NOT incense

if people did complain just say it is for religious meditation purposes

however if you are in the habit of going to hotels and like the incense smell the other option is an oil burner

They could and there may be a fine; to be really sure you really should check with the hotel. All it takes is one complaint.

the hotels are called "smoke free" - figure it out.