I am sorry Lut. It is difficult to figure out WHO!

My first thought was "Is Snoopy in heat?" As she gets older she may do more urine marking during her heat cycle. Just another possibility.

Although, banning Zazou from the garden may very well be the cause.

If it is an infection the cat will attempt to use the box often (more so than normal) so be observant and you may be able to figure out who.

Prayers to you that the answer comes (and trust me, I know how hard it is with multiples)