The most probably cause is the o ring seal on the reservoir this is available from Lexus at around $6.00 Part # 90080-30040
You need to remove the low pressure return pipe and 3 bolts holding it to the pump. ( you don't have to remove the pump itself)
Remove as much fluid as you can 1st using a rubber bulb type chicken baster then put plenty of rags under the pump to catch any remaining fluid.
Here are some pictures showing the bolt locations and the o ring.
Reassemble then top up the ps fluid and bleed the system.
Just wanted to thank you for this suggestion. I think it's going to solve the issues I'm beginning to have with my pump which was just replaced a couple years ago. The car leaks a small amount of fluid after it is turned off and left alone, and I suspect it is coming directly from the pump. The system does not "whine" while in use or act up, but i know P/S fluid bis leaking somewhere because the reservoir needs refilled routinely.
Hopefully an O-Ring is all I need, but are there any other things I should be looking for?