OK, I found 511AC talking to Grissom as he was going into Marion. Forgot that Marions is UTC -4 this time of the year...
KGUS-Apr-02-2018-2030Z.mp3 from liveatc, (I can't post the full link yet).
511AC was cleared to desend to 2500, and instructed to report Marion in-sight.
at 27:18, or just a few seconds before, 511AC reported airport in-sight
ATC offered to cancel IFR, or wait till on the ground,
511AC cancelled IFR.
Tower said "No traffic between you and the airport, cancellation received, change to advisory frequency approved"
511AC acknowledged.
That would have occurred about 11 or 12 minutes before the time of the reported incident.
So without the recording from 122.7, there is no way to know what happened, except that the airport manager or someone had reported that the C150 pilot did broadcast intentions, I read that in one of the news articles but can't confirm it at this point.
I am thinking with a 14 knot headwind, the C150 would not even think of looking down the other runway.
also, I am wondering if there may of been other confusion for the c150 pilot, with the winds he might have heard some one say on final straight in or something and thought
the plane was behind him I think runway 15 is a "BACKTAXI" runway with no taxi-way. therefore he might have rushed himself to get off the runway, not knowing where the other plane was.
Of course communication and good radio skills would have corrected that.
Also the C150 did have a MODE S transponder (per registry info at FAA), and I wonder if he did not set it to ALT until after the runup, this would mean that Grissom already disconnected with 511AC, before C150 became visible from the MODE S.
A lot of checklists have the set to standby, until your runup is done, I am thinking about leaving it on ALT unless I am at a big airport from now on....
if it was set to ALT, there is a chance that Grissom would have seen the plane on the runway and been alerted to departing traffic on 15.