I had an odd experience with Schengen at the Gibraltar border earlier this year and am wondering about the implications of this.
I crossed on foot from Spain into Gibraltar. I first went through Spanish exit controls, which had biometric e-gates for EU citizens. As I held a US passport, I was waved around the gates and was never stamped out of Schengen, nor did anybody note or record my passport number. It is apparently Spanish policy not to provide exit stamps here, though they may do so if you're really insistent. This was followed by a small desk used by Gibraltar immigration, who glanced at my passport but also didn't stamp it.
For my trip, I crossed back into Spain a few hours later, having enjoyed a lovely hike down the Rock and seen my fill of wild monkeys. Again, no Schengen entry stamp, and my passport number wasn't noted nor entered into any kind of computer system.
What I'm wondering though is how this would have worked had I instead flown out of Gibraltar by air. I understand that Gibraltar is not part of the CTA, so I would have had to go through UK immigration, but how would Schengen know I left? I would have Schengen entry stamps in my passport (from when I flew into Spain in the first place), but no exit stamps, as I wasn't stamped out when entering Gibraltar. Presumably, if I later turned up at a Schengen entry point, they could think I overstayed.
And similarly, what if I began my trip in the UK and flew to Gibraltar. Could I have then crossed into Spain and wound up in Schengen with no entry stamps or computerized record of my arrival?
Is this normal? Does it break the Schengen entry/exit tracking system?