定语从句she was looking for her bicycle key( )she thought she mi

这个不是定语从句,因为定语从句要有先行词,at which 明显表达的是地点含义,前面必须有与地点相关联的先行词.这是一个地点状语从句,应该填上地点个状语引导词where,表示在哪里找钥匙,意思是正在她以为可能早先放钥匙的地方找.
例如：
She kept the picture where she could easily see it.(她把相片放在她容易见得到的地方.）
She told me something about the police station at which her father worked 3 years ago.( 她告诉了我一些她爸爸三年前工作的那个派出所的情况.）

she was looking for her bicycle key(where)she thought she might have put it .
这不是一个定语从句。