Assuming you do all eight non-Boss rooms during a delve run? Also assuming that it’s 15% for one of the offers to appear after a non-Boss battle? (I think that’s the number advertised in the patch notes.)
You’re not quite 75% to get at least one Offer during a delve run. So it’s plausible that you could spend three sigils, do all 24 non-Boss rooms, and never see one. It’s not too terribly likely, but “bad RNG” can land you on that approximately 1-in-64 odds of it happening. (It’s actually a little less if my math is correct, namely 0.85 ^ 24.)
You’re also roughly 60% to not see an Offer in a single Explore run, using the similar baseline percentages.
I just used my two daily sigils on a deep delve and got the Daemon offer after the 2nd battle. So they can show in deep delves, but only outside of events. In events, they won’t show up, whether it’s a deep delve or not.