Lots of pagers operate in one-way only mode. Towers transmit messages without expecting acknowledgement a few times, pager is configured to filter out and only alert on messages routed to its ID.
Sure, theoretically one can detect a receive-only radio, but its massively more difficult than detecting something which actively transmits.
While the messages are not encrypted, you just have your actual message coded. Have agreed on phrases and what not discussed out of band. Send dummy messages to throw people off and not know what is a real transmission or a dummy one. Is that numbers station just spouting gibberish or communicating with spies?
The market closes at 5, dinner at the hotel, Grandpa will bring home the wine, bring your hat. Charlie 5 Alpha 2 4 7 3 Bravo. Maybe this is just discussing someone's evening, maybe its coordinating a group action.
Most pagers do, yes. They are also usually unencrypted. And due to the one way nature, even if they are encrypted, PFS (perfect forward security) is impossible. Meaning that if someone captures the encrypted messages they can decrypt them all the way back when the encryption key is obtained.
But the impossibility of any kind of location tracking is definitely a plus of one-way pagers. Not just for terrorists. I'd get one if there were still a network where I live. It'd be really nice to be reachable and not be tracked 24/7 for once.