We have users go into moderation almost daily. I'm guessing that most of the time they will be people whose IP or email address is on stopforumspam. But there is no indication as to why they are in the moderation queue, and this is quite an important omission requiring work by the moderators to figure out why the account is in the queue and what to do with it.
How do you think we could resolve this please?
I can only suggest to contact them, and simply query them (put their IP or email address into search engines) or search each of the APIs. It would be nice to have more data when users a put into moderation (so if we are picking up false positives from particular APIs, the core can be tweaked).
As Chris mentioned, putting users into moderation is core behaviour, and since they have got passed FBHP they are unlikely to be bots. From my own experience, they've also not been spammers (but that is across a only a few forums, none have daily user moderations).
However, StopForumSpam (and other APIs) will stop human spammers, FBHP will not. So, it is possible some of these will be
human spammers or
false positives. If you have that many, google a few, it might just be that StopForumSpam is picking out the human spammers in your case.
In my experience, APIs are definitely a useful broad catch (one that is very hard to bypass), and may outlive many mechanism (it may one day be our only resort... although there are ideas to bypass even APIs)
But, the more APIs are used in combination, and the more weight they are given, the more false positives are picked up (they can be noisy when relying on human reports)
It would be a good idea to see why/when the core has found false positives (by putting users who are not spammers into moderation) and to be able to tweak the APIs in accordance. Although defining "users who are not spammers" accurately might be harder than 1st thought. Spammers tend not to reply to emails, and re-join the community with a positive outcome (this is not a hard an fast rule for previous human spammers, we are very assorted)