Could aliens be sending signals we can’t decode?

Yes, it’s possible in principle. We don’t know whether extraterrestrial civilizations exist, let alone how they might communicate, so some signals could be missed, misidentified as natural phenomena, or be hard for us to interpret.

A few reasons this could happen:

• They may use a signaling method we are not looking for, such as a narrow radio band or lasers, or something much less familiar to us.
• Their message could be very brief, very rare, or sent on timescales we are unlikely to catch.
• The signal could be encoded in a way that looks like ordinary natural variation or human-made interference if we don’t know what to look for.
• Even if we detected it, we might lack the shared context needed to understand it.

That said, this is still speculative. Humanity has not confirmed any alien signal, and many candidate detections would turn out to be natural sources or human interference.

One caveat: ideas like perfectly “compressed” messages being indistinguishable from randomness depend on the details of the encoding and the background noise, so that point isn’t a simple yes-or-no claim.