Not necessarily anything!
I just ran a few test cases of Land Strike missions against various SAM sites with F-16CM Blk 52s w/ HARMs. I noticed that they only engaged with HARMS when the SAM site turned on their tracking radar, not just their acquisition radar. In the case of an SA-13 this meant they wouldn't engage with HARMs at all since it has an acquisition radar but missiles themselves are optically and/or IR guided. That might or might not be a bug, depending on how you think about it. Maybe one day the "Radar" category in the WRA might be split into "Search Radars" and "Tracking Radars" to enable a little more nuance in some cases (e.g. SA-13)?
In the mean time, it looks to me like they made a decision about AI behavior with ARMs and assumed (not necessarily wrongly) that you really want to destroy the tracking radar in order to cripple the SAM site. If you only destroy the acquisition radar a SAM site might still launch. Therefore, to take advantage of the maximum HARM range, you need to bait them into illuminating their tracking radars. Where the SAM site lights up his tracking radar depends on a few things including terrain, WRA and proficiency settings.
Is the emission you're detecting a tracking radar or an acquisition radar?
I have flight Jimmy95 on a land strike mission against an SA2 site. I have both the side WRA and the unit WRA for Harm missiles set as shown in the lower right corner. I've verified that the contact is emitting, and yet the flight won't take a shot at it. Any ideas as to what I'm doing wrong?