Mortal Durkon was a Big Liar!

[thereaper]

Did the stat bonuses from the vampire template give Durkon more (empty?) spell slots?

Technically, yes. Vampires get +2 Wisdom, which would give Durkon at least one bonus spell slot (possibly more, depending on what his wisdom score was before). What level the new slot is doesn't matter, because Detect Magic is a cantrip anyway, and you can prepare a lower-level spell in a higher level slot.

Of course, choosing to prepare Detect Magic in your new bonus 6th or 7th level spell slot might seem a little odd, but Durkon has always been terrible at spell selection.

This, of course, is all assuming that the LG hasn't stopped to rest. We don't know exactly how long the Order was out of commission.

Edit: And now that I look closer at the bonus spell slot table, at Wisdom 20-21, 22-23, and 24-25, you get an extra bonus 1st, 2nd, and 3rd spell, respectively. Given the situation, it would actually make a lot of sense to put Detect Magic into one of those.