We got back last night from meeting some old friends for dinner, and as soon as we walking into the house, ladysolly commented "There's a smell".
My nose is useful in inverse proportion to it[s size, or to put it another way, I can't smell much of anything except curry. But I could detect a faint aroma. So we went round the house sniffing, and found nothing.
In the morning, I went into the data room, and the smell was stronger; after a lot of sniffing around, it seemed to be coming from one of my UPSes. These are, essentially, a lot of lead-acid batteries and some electronics. If a battery goes bad, it can start giving off hydrogen sulphide (or sulfide as the Americans would have it), which is the bad eggs smell that we all know how to make in the lab. So I powered off that UPS, and switched the servers attached to it, to a UPS where I'd recently replaced the batteries.
Of course, one of the servers didn't come back up when powered on. I opened it up, checked the tiny CMOS battery, and that was dead, so I replaced that. Then I was able to get that server going.
I'll wait until the UPS cools down, then I'll take it apart and have a look inside. I replace the batteries in that smelly UPS in 2013, so I'm a bit disappointed that it's only run for two years. I'm expecting to find at least one faulty battery, and I'll replace them all.
... later ...
Several batteries were severely distorted; it was so bad that I had to use a crowbar and mallet to get the eight batteries out. So I've ordered a set of replacements.