Gus did the same thing. He would go sit silently by the door but if I didn't notice immediately he would pee inside. I taught him the bell trick but he would never ring it to tell me he needed to go potty. He constantly wants to go outside and would only ring the bell after we came inside and he wanted to go back out.
I didn't know what to do so I just had to watch him closely. Around seven months old he did start to cry when he needed to go out so maybe it just takes time for them to figure it out. It's not a solution but maybe some hope.