Told that have previously overstayed when haven't?

A friend of mine with a UK ancestry visa (obtained in 2015) has entered and left the UK with no problems since. Obviously her visa and BRP is valid until 2020. Yesterday after returning from a short trip she was kept for "extra checks" and told she had overstayed in the past by a few days. They said it didn't affect her current stay and she's allowed to stay and renter the UK but this will pop up in the future again. She is very confused about why this happened as she is certain she has never overstayed, before the ancestry visa she has only ever entered on a tourist visa which is a minimum of 6 months and she has never stayed in the UK more than a month or two at a time prior to her ancestry visa. She told the immigration officer this who advised to write to the home office. She asked if she could know the dates that they think she overstayed so she can check but they refused to give her this information. Does anyone have experience or advice with things like this? She is very confident that she has not overstayed in the past especially as she has left and entered the UK multiple times since getting her ancestry visa and this has never happened until now and it would not have been possible for her to overstay on her ancestry visa as it is valid until 2020?

Not possible to overstay on current ancestry visa if leave is valid until 2020. It may be related to her previous visitor visa. But it may also be their mistake.

Suggest SAR.

I spoke to her since and she said that the immigration officer said he believes her and advised her to speak to the home office to avoid further problems. Her biggest concern is that it will affect her eventual ILR/Citizenship applications. She also said he mentioned it was in the last 5 years and other than her ancestry visa she has visited once for 10 days in July 2013 and that visa was valid until September. I imagine it is most likely a mistake but a very odd one indeed!