12/13/67:
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..Government sources acknowledged that the Administration was considering giving field commanders in South Vietnam authoriy for "hot pursuit" into the country."
...The present interpretation of the policy of self-defense generally bars hot pursuit, but in practice American commanders have engaged in it on occasion during the heat of battle."
12/12/67:
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...a plan to this effect, allowing for follow-up by American forces, was "under active consideration..." ..."the decision to move into Cambodia was all but made."
11/16/68:
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Since last July Cambodia has been holding eleven American crewmen from an Army river supply vessel that strayed inadvertently into Cambodian territory.
...Cambodia has finally recognized the two-sided nature of the border incidents and has asked the International Control Commission to investigate the Communist incursions that provoked allied attacks across the border."
1/22/68:
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Cambodia said today that equipment was abandoned on the battlefield by the "American-South Vietnamese" force that, Cambodia maintains, crossed into her territory Thursday....
...He said that the abandoned items included red scarves worn by paratroop commandos, a United States officer's helmet, weapons, and radio sets.
Cambodia regularly charged that forces attacked targets, military and civilian, in their territory throughout this time period. I'll leave it to contemporary scholars to unravel the truth/lack of truth of those claims. But, it's quite clear that at this time the US was in possession of quite a bit of intelligence about North Vietnamese troop positions and strongholds in Cambodia which would have been difficult to derive without some cross-border surveillance.