Taking the original post at face value:
Glock 17 mag capacity has been addressed.
Method of carry in terms of 17+1 or 16+1 has been addressed.
Department policies regarding such would be a factor in an investigation of an officer-involved shooting, if any exist.
My department required full-up loading in all Glocks issued or private purchase, largely to avoid this type of post-incident "Did you fire one, or did you fire two?" issue.
If the Glock (or any other pistol) is carried routinely full-up, it can help reduce uncertainties later on in reconciling brass found at scene, number of rounds found in gun, and officers' or witnesses' memories.
That said, I've been outa there long enough to be able to say today that I took into consideration the tendency of Glock 17 mag springs to shorten under constant tension, the single mag-related Glock 17 feed failure I personally experienced (the only failure or stoppage in that 1988 Glock that I still have today), and deliberately downloaded my carry mags by one round each in clear violation of written department policy to reduce the possibility of a shortened spring causing a malfunction.
In the years since retiring I've intermittently carried a Glock 17, always downloaded by one round.
My own reasoning, which I do not promote for others who can make up their own minds, was & is that I prefer to be around later to explain the number of rounds carried/shot, rather than risk lying in a puddle of red with 18 rounds accounted for and a jammed pistol, at which point how I started out would be a literal dead issue.
In saying this, I do not mean to start up the usual arguments that mag springs can't shorten during long periods of constant compression.
I've seen it personally IN GLOCK 17s, and I offer the above commentary to address the original question posed about carrying one.
I also do not mean to suggest that stoppages from shortened mag springs are a high risk factor in Glocks.
Shortened mags was a known factor at my PD, the armorer would switch out old for new if he found a significant shortening in carry mags during his annual inspections.
I was not the only one who downloaded by one because of it.
Possibly unnecessary, not interested in debate, merely offering the above as a reason why a 17 might be carried downloaded by one.
Also, during a shooting where adrenaline & chaos run high, it's quite possible that those present may not remember if one shot or two were fired.
Denis