1) Transmitting on one channel would be easily visible on spectral analyzer, but if you will transmit on multiple channels (Frequency hopping spread spectrum, like Bluetooth does) on some arbitrary frequencies, there is no chance to see it on spectral analyzer, unless you know where to look.

2) You would need something like a dedicated SIGINT drone which will be capturing and sending this data stream to HQ to process it, which itself would be visible like a flare in the night sky. I am not aware if Russians are using any SIGINT platform and if they do it is not publicly known. Ukrainians are having NATO SIGINT planes flying around Ukraine and will definitely have some ground based SIGINT devices from Western nations. For example we know that Ukrainians are constantly intercepting drones with interceptor drones but it is kept secret how they are actually able to find the drone in the first place.