I have had this happen several times. I still secondary all my beers because I like more work and cannot justify being more efficient. No, seriously, I brew in buckets and use the secondary process to free up buckets, allow a little more aging and clarifying and because I am still excited about seeing part of my beer before I put it in a glass. Going from bucket to keg just seems too “production oriented” to me.
My normal process is that I brew, primary goes normal, control temps in low to mid 60’s, generally two weeks minimum in primary. I rack when I can tell it is done. I had been doing extract and sometimes I was not getting my FG as low as I would like, but when the foam drops and the SG is stable, I rack to secondary, generally between two and 3 weeks. Secondary is generally a week or so for normal beers, then I cold crash and keg if I have an open keg. Often I have to wait a little longer to wait for a keg to kick.
For the last several months I have been brewing double batches, meaning two completely different brews in one session. After racking to secondary (I know it is not too early) I let the batches condition at room temps (65-75). This has happened 3-4 times now. One of the two, after about a week in secondary, with stable warm temps, will start to bubble again. Then a thin layer of foam develops and keeps growing. It is slow to grow but it does increase. The two brews that I have in secondary now are extract version of the Midnight Beatdown Wheaten Porter and Waldo Lake Amber. I think it is the Porter that this second fermentation is happening to.
In the past it has happened to a Caribou Slobber and an Oatmeal Stout. The Caribou Slobber (keg just kicked) was my second batch and was clearly different than my first. Less sweet, and seemed “thinner”. Very little mouth feel. Just like something was missing. The Oatmeal Stout is really good, but is not at all sweet, not that I expect sweetness from an oatmeal.
I am really curious what is causing this secondary fermentation. It seems that this has happened with different yeast strains. I am pretty (100%) sure it is not an infection. Should I let it go forever or cut it off when this starts by cold crashing?