European doctrine (I think from the Nazis) was that pistol magazines should NOT drop free as ammo would be easy to find on a battlefield, while magazines would not be as easy to find, so the first Glocks were made to generally not drop free.

As Mr.-wilson states this is not the procedure with US police departments and glock started making metal lined mags that would more reliably drop free, though Glock does not guarantee that even the metal lined mags will drop free. The metal lined mags have a square cut out at the top rear of the magazine (where the primer on a round is located if there is a round in the magazine) on the non drop mags the cut is round. You can see the metal lining through the holes next to the markings for the number of rounds in the pistol anyway.

Most non metal lined mags drop free if they are not completely loaded. You can sometimes get non metal lined mags to drop free by spraying Armor All on them. The non drop mags will still come out a little bit, then you rip them out.

With 18 rounds I don't think it's too likely to come up, but I think a spare mag is worthwhile in case the magazine in the pistol malfunctons.