Is an arbiter allowed to leave the playing hall while some games haven't finished yet? I've observed the arbiter responsible for the category I'm playing often leaves the playing lobby to smoke or drink/buy coffee/water in a nearby shop. Those &quot;breaks&quot; are quite long. What happens if both players run out of time/need another scoresheet, etc.?
Is this behaviour acceptable? What should I do?
The first thing to note is that for low level tournaments there is not even a requirement for a qualified arbiter to be on site. The arbiter must be a licensed arbiter (International Arbiter, FIDE Arbiter or National Arbiter) but is not required to actually be there. They can be at home watching TV. If there is a problem they can be consulted on the phone.
For reference here is what the FIDE Rating Regulations effective from 1 July 2017 have to say about arbiters for FIDE rated events:

0.3 All arbiters of a FIDE rated tournament shall be licensed otherwise the tournament shall not be rated.
0.4 Tournament reports for all official FIDE and Continental events must be submitted and shall be rated. The Chief Arbiter is responsible for the results submitted.
9.1 The Chief Arbiter of a FIDE registered tournament has to provide the tournament report (TRF file) within 7 days after the end of the tournament to the Rating Officer of the federation where the tournament took place. The Rating Officer shall be responsible for uploading the TRF file to the FIDE Rating Server not later than 30 days after the end of the tournament.

No mention of the arbiter actually having to be there and some federations run FIDE rated leagues where there are two or three licensed arbiters for the league but dozens of clubs where the matches are played with no arbiter present.
For Norm events (tournaments where there are enough highly rated and titled players from enough different federations for it to be possible to get a title norm) the rules are a lot stricter.
This is what International Title Regulations (Qualification Commission) / FIDE Title Regulations effective from 1 July 2017 has to say:

1.16 The Chief Arbiter of a title tournament shall be an International Arbiter (IA) or FIDE Arbiter (FA). He may appoint a temporary deputy. An IA or FA must always be in the playing venue.

Let's just remind ourselves of how the FIDE Laws of Chess define &quot;playing venue&quot; and &quot;playing area&quot;.

11.2.1 The ‘playing venue’ is defined as the ‘playing area’, rest rooms, toilets, refreshment area, area set aside for smoking and other places as designated by the arbiter.
11.2.2 The playing area is defined as the place where the games of a competition are played.

So, even in a high level norm tournament the arbiter is allowed to leave the playing area and go to the toilet, go and have a smoke, go and get a coffee.