Planning to do a DIY night tour of Doha by following this old
post, except I would be storing my carry-on and (try to) pre-book a sleeping room @ AS. How does this part work from AS instead of AM ?
From there you now need to get back to Arrivals, and this is the bit that is not signposted. But what you need to do is go to Level 2 - exit the Lounge and take the Lift (not the escalator) down to Level 2, where you will be arriving on the output side of transit security. Here you need to speak to someone and explain that you wish to go through to arrivals - you effectively need to walk through the transit security scanners the wrong way (against the flow of people) so just speak to someone as you do this.