Residual trans fluid in coolant?

2004 Jeep Grand Cherokee Overlander. About ten months ago I had to replace my transmission. I had a rad failure - coolant had mixed with trans fluid and trashed it. There were no signs other than low trans fluid and then a code for internal failure. We didn`t know why the tranny failed til it was opened up and voila pink milkshake. So we replaced the rad as well, coolant system flush was done at this time. Now I go for an oil change and they point out my coolant in the overflow reservoir is dark purple and there is oil floating at the cap site. However the tranny fluid is not down at all and is the correct color. So is the tranny leaking into the rad - hence second rad in less than 12 months, plus coolant and tranny flush and a prayer. Or is this residual tranny oil that was leftover from the recent failure Mechanic recommends flushing again, including reservoir removal and clean and watching for re-occurence. They said pockets of oil can hide in the heater core and the work that was recently done this could be residual oil, not new stuff.