Our steam driven AR15 (RIP
) use to see balloons even though it had MTI (moving target indicator - the thing that gets rid of permanent echoes). Gliders and microlights weren't a problem too. I once gave traffic information to a B25 (yep, a B25) that was landing at Bovingdon for filming when the pilot announced he had the traffic in sight, it was 'a large model airplane'.
I'm sure I have seen balloons on our ATM, and the police heli shows up when it is doing not many knots.