Strange happenings on arrival at MAD

[Traveller]

I flew LHR-MAD-LHR this weekend. Fairly uneventful trip - checked in
online, arrived LHR at about 1630 on a Friday and it was a breeze, T2
security in < 10 minutes, and then off for IRIS enrolment. To my delight
there was only 1 chap in front of me, and the whole process took but a
moment.

Pleasant bite to eat at Caviar House (not cheap, but fairly good) and then
off we went. Pretty much ontime departure and then arrival into MAD's new
T4, which I have to say is a fine building. So we spill off the plane and
head for the exit, wondering where passport control was going to be.

Next thing we're in the baggage hall, thinking "that's not right..." and
then, with hand luggage only, bang, we're landside.

Yup, no arrival control. Now I know Spain has brought in the API and this
means they get a good 2 hours to have a look through the files and see who's
on the flight, but still I am amazed they treated it as if it were an
intra-Schengen flight, especially as there were non-EU nationals on board
[or at least I assume so as they asked for landing cards when they were
being distributed].

Thoughts on why this was, anyone?

BTW I had a good trip back, ontime departure and arrival - I guess LHR does
work sometimes - and IRIS saved me time for sure on re-entry to the UK, with
fairly long queues in the passport hall and only 1 person in front of me at
the barrier. The system struggled a bit to recognise me until I took the
specs off, and then there was quite a long pause while it looked in the
database.... but all was well.

Note to self I: Don't assume your flight departs from the same part of
terminal 4 in Madrid as your friend's earlier flight.

Note to self II: Check where your flight is leaving from sooner than 40
minutes before departure

Note to self III: Be thankful security and passport control weren't busy as
you ran like a loony from T4S [which is a satellite of T4 in MAD used for
*some* - but NOT all - non-Schengen departures] - to T4, including a frantic
wait for the automatic shuttle train which is a much longer journey than you
would have expected. Gasp. Wheeze.

T

[Traveller]

A mistake? Seriously, buses have gone to the wrong bit of Schiphol
before...

That is, of course, a possibility, but I would be very worried indeed
at such a mistake happening. Imagine the fuss in the UK if a
planeload of pax from Spain managed to transit LHR without being
screened? I was always pro-Schengen until now, but if this sort of
thing can happen then I'm not so sure...