We saved a swarm from our city water cover and moved them into a basic hive we built. They seemed to be doing great, but today they don’t seem to be in the hive. Could they just be gone temporarily? They were a great swarm, so docile. 
How long were they in the temporary hive before they disappeared?
Gone, as in absconded?
Not absconded. May 7th evening they were moved into the hive. On the 8th, we took them a few miles away for a couple days as we had been told to do so they wouldn’t return to the water cover. We just checked on them this past weekend (16th) and my husband added more frames. They seemed to be doing great and tonight they seem to be missing.
Sometimes swarms still have an urge to continue swarming. It has nothing to do with you. I have heard of beekeepers placing a queen excluder below the bottom hive body to keep the queen inside. Once wax is drawn and the queen lays eggs the swarm will stay. Just another technique for you to set aside for future reference. So sorry they took off.
How does having a queen excluder below the brood impact the hive? Does it impact the longevity of the bee’s lives? Are there any other pros and cons? I presume it doesn’t prevent the desire to swarm and so you need to split the hive if there are signs they want to.
I’m just thinking it might be a good prevention of swarming in a built up urban area and put the council’s mind at rest? I’m introducing a nuc in the spring and so won’t have the problem of an unmated queen.
The bees can still come and go as normal as they fit through the Queen excluder. She is the only one blocked from leaving. And yes, you only do this with a mated queen. I haven’t used this method yet as the swarms that moved into my bait hives have stayed. I use the queen excluder on top to collect propolis which I harvest. A queen excluder should be constantly monitored especially when used to hold a swarm queen in.
Can you use a method similar to installing package bees? ie the queen is in a cage for several days allowing the hive to get established? Or is this typically unnecessary with a swarm like this? How common is this kind of abandonment?
A Requeening Frame is what I prefer to use. The queen cage fits into it and gives a swarm prone queen space. I rarely introduce a new queen to a hive but when I do I use the Requeening Frame.
It is fairly frequent that a swarm will visit several locations before they finally settled. Swarming is very much part of their superorganism way of reproducing.