The Irish Horseracing Regulatory Board is investigating the betting patterns surrounding two races at Leopardstown on Saturday.
Melon drifted moments before the start of the BHP Insurance Irish Champion Hurdle in which he eventually finished fifth, while Yorkhill was easy to back before coming home sixth in the Dublin Chase.
IHRB chief executive Denis Egan believes both races merit further examination.
He said: "It's just suspicious betting patterns we are looking at, it may be nothing but we are investigating.
"There were some lay bets in two races we felt required further examination.
"We are awaiting the relevant information and we hope to have that in the next couple of days, then we can decide whether to investigate further."