But its all based on context that you, as the DM, are responsible for creating anyway.
Maybe the small town is very rich with a well funded criminal justice system, so their locks are better than average. DC 15 or 20. Maybe the king's vault was damaged during an earthquake (which may or may not have been summoned by the party cleric). DC 5, and the trap is broken.
The point is, what you should be asking is how hard do you want it to be? The ease or lack thereof should be contributing to the overall situation that youre trying to create. If the players take some preemptive steps to make it easier, great! Take that into consideration. Advantage and disadvantage aren't the only tools you have to represent context making a task more or less difficult.