How should I approach this mistake by the dealership?

Six months ago, my local mechanic found a transmission leak on my 2018 Sequoia and told me to take it to the dealership and have the work done there since it was still under the 60k miles warranty. They got the leak fixed but somehow dropped something into engine which required the dealership to disassemble the engine and remove the foreign object.

Around May when the weather started to warm up I noticed the engine temperature would rise when I was idling for 5 minutes. Checked the coolant levels and it was at the full line. Had my mechanic looked at the issue and he didn’t find a leak. The only thing he could figure was the temperature sensor was faulty and wasn’t opening the valve for the engine coolant system.

Wellllll two days ago the engine temperature sensor was on the high side again. Dig some further digging an found this sign in between the radiator fan and radiator coil. Yesterday I went to the dealership and found same service rep I dealt with before. Explained to him what I found and he said “oh our guys will tape a sign there to block the fan when working on the engine. I guess they forgot to remove it”. He took me to his boss and I explained again the situation and what I found. I asked him how are you going to make this right because this sign has been there for six months / 10k miles. He set me up for a visit later this week with a rental car and told they will do a full diagnostic of the car. At the end of the conversation he asked me to give him the sign which I said “nope the sign is mine and we can talk about this when I come back this week”

Tomorrow I going to see my local mechanic to give him an earful why didn’t he find this when he had the car last month or in April when I got the oil changed. His company likes to have the car a full day to do a full inspection.

What do you think is fair to ask the dealership and local mechanics for this? I was thinking the dealership pays for all my maintenance till 150k miles to ensure there are no further issues due their mistake and getting my money back from the local mechanics when they replaced the sensor.