Willem II striker Fran Sol was at Vicarage Road on Saturday.
Sol took in Watford's 0-0 draw with Crystal Palace, confirmed Brabants Dagblad, sparking talk of the Spaniard preparing to move to the Hornets this summer.
The 26 year-old has scored 16 goals this season and is tied to Willem II until 2019.
Along with Watford, Sol is also interesting Valencia and Villarreal.
Watford and Willem II enjoy a positive relationship, with Hornets sending Obbi Oulare to the Dutch outfit on-loan last season.