The 5/8" pin I use DOES have the hole, not the groove.
I do a walk around EVERY time we stop for fuel or whatever. Adamant about it.
I caught a "line" in a shackle on our trailer at the LAST stop north of SLC before continuing to Idaho. one trip coming home from Moab, 70 miles of nothing past this place but blowing dust warning signs... (seriously- the State has 3 miles of trees planted for a windbreak it's so bad). So I see this line and reached between the trailer tires and sure enough my fingernail caught on the crack. This was at a gas station. Happened to be a farm store 200 yrds down the street and that was a lucky but smart 45 minute fix 20 minutes before the store closed for 2 days!
Our motorhome can barely go into gas stations much less catch some brush to pull the spring clip- I understand what you're saying for sure- HOW could this have happened. My long explanation in the post after it happened a couple yrs back did mention how every time I slid the 5/8" pin in to one side it would always slide towards the DS so the spring clip was rubbing against the stinger. After 3 times of seeing this, I pulled the 5/8" pin from the stinger/receiver at a stop and slid it in from the OTHER side, pushed the spring clip thru and proceeded to stop in about 40 miles JUST to check it and sure enough, the 5/8" pin slid the same direction as it had every other time except this time it was up against the 40deg angle at the end of the 5/8" pin instead of up against the spring clip.
Next stop I bought a locking 5/8" pin so there couldn't be anymore accidents like this again... whatever the cause! I really can't believe someone would remove the spring clip (maybe so but

dang) but do believe that whatever caused that 5/8" pin to ALWAYS slide to one direction caused it to wear or somehow catch on a safety chain??? and somehow come loose but we were soooooo freakin' lucky we didn't have a serious disaster. whew...
I mean the motorcyclist caught a cement chunk that the trailer ball was grooving into the concrete at 65 mph or from our hitch OR maybe the 5/8" pin into his chest and he dropped to his knees as he got off his bike and pulled over. Had a big dent in his gas tank also. This was all at 65mph in 8 lanes of traffic going in ONE direction.... HATE driving thru SLC... one year we saw a semi that had tipped over and crushed a person to death in their car... just blew the semi over. They closed the freeway just after we made it thru in our streamlined motor home! lol
I wrote a book didn't I??? lol
I'm going to guess the clip was somehow removed whether by a bush on the trail out or by a grumpy person at a gas stop. It's fears of dropping a pin that makes me check that it is still pinned and clipped as the very last check after the gas is pumped and I'm about to jump in the drives seat.