Replaced spark plugs why not mention timing cover leaking too

I brought my 2010 Prius with 172,000miles in as I was having problems with it. The dealer three weeks prior said need spark plugs and fuel injector clean on a routine oil change. But after yesterday doing the spark plugs instead of fuel injector clean it's billed as throttle service. Then on back page says my Timing Cover is leaking oil? My question why didn't the dealer tell me about the timing cover during this repair. I feel like it's a way to make me return for more $$$. Am I being unreasonable, I feel I'm being scammed.