If you look here: https://fr.wikipedia.org/wiki/Futur_ant%C3%A9rieur_en_fran%C3%A7ais (yeah OK worst source ever, whatev'), you can see four uses:
- Something that will end for sure in the future
Il aura terminé ses devoirs dans une heure.
- Something in the future but before another thing
Quand j'aurai terminé mes devoirs, je viendrai manger.
- A supposition
Il n'est pas encore arrivé ? Il se sera arrêté en chemin.
... equals 'il s'est probablement arrêté en chemin', the futur antérieur is already a supposition in this use case, and I would add that the probability with the futur antérieur is high, it's not maybe, it's probably. You can still use peut-être, probablement, sûrement to precise the degree of probability, but without any of these words, there is a notion of high probability, in my opinion.
- An assessment, a summary
Sa vie entière n'aura été que joie et bonheur.
In your case, it's a supposition with a high probability, but indeed I feel it like it's another case not explained on Wikipédia: since there is a notion of probability in 'a pu', the sentence sounds even more like (not the translation but the meaning):
If he really used it (we don't know what 'it' is in your example) to take the bag, so he needed a partner waiting for him downstairs.
I feel it like: MAYBE he used it (the probability is here), and if so, he NEEDED a partner (no probability, it's a consequence of the first probability if this probability is true).