If you've ever played with an IR camera, there's plenty to be seen. It would be quite hard to have a balloon without significant contrast with the background of the sky. Especially if it had running electronics. Consumer sensors can detect a fraction of a degree difference.
The pico balloon's electronics run so little and so infrequently, they're not likely to be generating noticeable heat. The solar panel itself, a big black-ish object just hanging in the sun, would be nice and warm just like an equivalent rectangle of black paper would, though.
I think the envelope is gonna be the much more visible target. At high altitude silhouetted against the blackness of space, the balloon itself will be considerably warmer, even in what we consider "cold" air. Or if you're looking down on it from above, silhouetted against the warm ground, it will be quite cold.
Also the mylar, which I presume is aluminized, is a heck of a radar reflector.
Air at 30,000 ft is -50 F. You need, even for a very small box, heating measured in watts to keep your electronics at a temperature where they'll function. Even if you didn't, even tiny amounts of heating are going to be visible. The electronics box will look like a lighthouse beacon in IR.
Source: have launched small electronics to 100,000 ft on a weather balloon.
> These floater balloons often use only solar panels, no batteries. Batteries were dropped from the projects early on because they have limited charging cycles before they stop accepting a charge, especially in the harsh temps at altitude, -40F/-40C or worse. When the battery stops accepting a charge, it ends telemetry from the mission. So they only report telemetry during daylight, when the sun is at a high enough angle to illuminate the tiny solar panels. In the Arctic winter, the days are short and the sun might not get high enough to wake up the electronics.
