ETI air conditioning and spray deodorants?
In a related topic, there was a buzz a while back in the SETI community over "Technetium stars". These were stars that showed the short-lived radioactive element Technetium in their spectra. Since this element has a very short half-life, any found in a stellar atmosphere should have decayed long ago. Hence, it was proposed Technetium was being manufactured by ETIs and deliberately dumped into their sun to advertise their presence.
Alas, it was soon shown that there are natural thermonuclear mechanisms that could continuously produce Technetium in stellar interiors and that it could be convectively transported into the surface layers where we could see it.