"Warm" is one thing. All wires generate heat, and it's perfectly normal- after all, they're designed to work up to 75-90C in the worst-case scenario. If all the other wires in the box have no-load, it may only seem bad by comparison. "Hot" under normal operating conditions is quite altogether something different. Abnormal heat in this wire could indicate a problem- poor connection or undersized wire, for instance. If you think it's an issue, I'd get the prybar the clamp a meter on there ASAP to verify it's simply due to loading on the circuit. Can you access the far end of the circuit?
If this is a branch circuit supplying a fixed design load close to the NEC ampacity of the wire and temps are only slightly elevated above ambient, I wouldn't worry about it. But if by "warm" you mean "I burned my hand", then you may have a problem.
