Ive also built two of those "doo hickeys". Mine are not that accurate but for a quick voltage checker it works perfectly. At 4.17V there’s nothing to worry about. The non existent self discharge on the EFEST is curious though.I like your 24 hour discharge approach @Blu_Marlin
I have one of those "doo hickeys" i got from ohm @johan and its still going strong. I think its pretty accurate. Fully charged batts in my vape setup usually sit for a few days before being used. What I noticed is that the Samsung "blue smurfs" discharged slightly more than the older Efests. After being fully charged to around 4.21V or 4.20V and sitting for a few days, the Samsungs would be like 4.17V whereas the Efests would stay at 4.20V (using the same doohickey) Always found that strange because the Samsungs were fairly new and the Efests about a year old with (I estimate) about 100 cycles in the Reos each.
I assume a drop to 4.17V is nothing to be concerned about? They still seem to perform very well.
I must check some of my brown LG ones....