Hi Steve,
It's a volume licence serial number for Adobe 2015 for Windows, but we're using the standard installation which always requires sign-in regardless of licencing method. The installation asks for the serial number, and then when we sign-in I believe it authenticates it. A tech made a mistake of not realizing that multiple PCs were using this serial number so didn't login to the account with the volume licence purchased but a second blank Adobe ID with nothing on it (I think they didn't want Adobe to accidentally authenticate against a volume licence instead of the one they entered). To clarify, the activations on the second account come from the same external IP address, just that this tech mistakenly signed into a different Adobe ID when activating Adobe 2015 on a few re imaged PCs.
Obviously we recognized the mistake and are correcting it where possible (logging out of the second account and then logging into the account where the volume licence certificate lies for the volume licenced serial number) but there's still ~4 PCs left that are logged into this second Adobe ID.
Also, we forgot to sign out of Adobe before wiping a couple of PCs and are concerned about if we (or Adobe support) can remove activations from devices that haven't phoned home in 90+ days or something? We haven't noticed any activation issues, just being cautious to avoid any down the road as we still have ~6 PCs we plan to reimage.