It would take a long time but if all the water evaporated out of the bowl then sewer gasses would be emitted into the room from the toilet.

That would depend on several factors

1- If the toilet is not properly vented it can lose its seal

2- If the toilet is on the same stack as one above it and someone flushes the upper toilet it can cause back siphon

3- Capillary attraction can cause trap seal loss

4- As Larry stated evaporation

5- Oscillation (wind effect) as a down draft from a vent terminal can also cause trap seal loss

I have heard something like that before. Just make sure you go and flush it occasionally to avoid any of that. Solve the problem.

By the way it is not just the toilet seal that needs to be replenished

All floor drains and fixtures need to have their traps filled with water to prevent dangerous sewer fumes from entering a structure