There are only 3 ways touching the + batt terminal on your alternator with a multimeter probe will cause a spark.

1. loose nut holding the batt wire eyelet connector to the B+ terminal and your probe moved it. Worth checking.

2. loose B+ post to alternator body causing intermittent connection to the alternator internals. (worth checking tho MUCH less likely. Wiggle it and inspect to see if the plastic washer is there. It's probably fine)

3. The MOST likely cause....is the test probe accidentally touched the alt case at the same time it touched the B+ terminal causing a short. I'll bet if you look closely youll find a small burn mark on the case of the alternator or your test probe.

4. Make that 4 ways... Just occured to me. Possible skinned or melted battery to alt wire meaning it was already near a sharp edge or heat souce so it didn't take much to bump it causing it to touch ground. Since I didn't see the spark and you didn't say exactly where it came from....I'm guessing.

My money is on #3. Tight work space - possible poor light- leading to operator error...

Try again.... Hold on a sec...lemme plug my ears just in case theres a boom... Ok Go for it....