Surge suppressors use MOV's to shunt spikes between the two places each MOV is connected to. In a good unit there will be one each for neutral to hot, hot to ground, neutral to ground. The MOV can be thought of as a resistor that does not conduct much until the voltage across it rises beyond a certain voltage. Then it conducts a lot. There is some intrinsic leakage in these devices. IF the MOV has absorbed surges over time it will start to deteriorate. This is normal. MOV's are not a permanent device; they are a sacrificial device. The little LEDs (when done properly) look at the wear state of the MOV by measuring leakage current.
These MOVs can also be found on the input of equipment. It is relatively cheap protection for a manufacturers electronics. So better equipment is more likely to have some of these (e.g., a good UPS, a server).
The ones from hot and neutral to ground could be your current source. Testing a MOV is probably outside of the scope of this sort of advice.
Doing as suggested above, put one device on at a time and see if you can find the one (or more) that are causing the problem. It could be the cumulative result of a little current from multiple machines.
If you find a machine that does it, have it serviced or replaced as appropriate. If the MOV is doing that it should be replaced. It has probably reached end-of-life and not protecting as it should.