KL and BA are both Amadeus system users, so any booking of either airline exists in a single system. However the security should prevent airline A from retrieving airlines B's PNRs, unless a security agreement is in place, typically where both airlines are in an alliance and thus service each others' PNRs - this is not the case with KL and BA.
If airline B has a segment (could also be a codeshare) in airline A's booking, that would also enable cross retrieval. As far as I know BA and KL don't codeshare.
The scenario you describe does not appear to fit the above, if you PM me with a screen shot I may be able to throw further light on it if you wish. I would suspect that there may have been a KL segment, subsequently cancelled.