Finding electricity source for new Ceiling Fan install?
To answer your question are 3 way switched always hot? YES, there are a pair of travelers and a common what a 3 way switch does is flip the power from 1 traveler to the other, so if the breaker is on there will be power in that cable.
The other possibility is phantom voltage as others have mentioned.
I used to commonly wire hot to the switch location then run x3 wire, this is still allowed if both switches are in sight of each other code will allow only 1 to have the common for the 3 way.
If I am putting in a new circuit with only a light fixture I still feed the switch location and run x3 wire so a light and a fan can be installed later the wires are there. In this case only one of the hot wires are used red or black.
With the light on the other wire having been capped at both ends will indicate a live conductor with a non contact voltage detector or even a voltage meter (unless low Z meter or setting).
The only way to know for sure is to put a signal on the wire and trace it but as mentioned this could be a traveler on a 3 way.