Yellow spots on some plants

I'm growing clones in an AeroGarden, so hydroponically, with constant lights and constant dripping irrigation.

I'm growing 5 plants and 2 of them have 1-4 small (1 cm diameter) yellow spots on them in various spots (noot just the edge). What could this be?

Sorry no picture, my camera cant take good close ups.

could be lots of things, without a pic its hard to say.

hot spots from tinfoil in an HID system
metal deposits from tap water
lock out or nute burn