If it's under 20Kgs, then you could meet them anywhere. You'll be depending on your own airmanship, your lookout, and the need for the ground operator to not endanger you. Much the same as meeting any other unknown flying vehicle in the open FIR.
If they are over 20Kg, then you shouldn't encounter them above 400' AGL. If you're operating down at that height or below, you are more likely to encounter some fast moving metal which is a lot heavier than that and will really spoil your day
