

Packet data has headers that can identify where it’s coming from and where it’s going to. The contents of the packet can be securely encrypted, but destination is not. So long as you know which IPs Signal’s servers use (which is public information), it’s trivial to know when a device is sending/receiving messages with Signal.
This is also why something like Tor manages to circumvent packet sniffing, it’s impossible to know the actual destination because that’s part of the encrypted payload that a different node will decrypt and forward.
Lots and lots of games, either boardgames or digital. Or a better computer.