Recently I remodeled our master bathroom. I converted the tub to a walk in shower. I moved the drain to the middle but didn't install a trap. The guy at HD told me it was not needed since there was a trap from the tub. All I did was extend the pipe over to the new drain. Well now that everything is done there is a sewer gas smell coming out. (I have confirmed it by sealing it up with packing tape and the smell went away) Why is the old tub trap not working? It is in the slab and under a lot of nice tile work. Is there anyway to fix this without tearing out my shower floor?

you were correct to not double trap the shower. you can also extend the inlet of the trap for 24 inches (under my code).
the trap can lose its seal by capillary attraction. a piece of string hanging thru the trap can cause this.
that's a wild guess but that's all i can think of..

After removing the tub, were you left with a horizontal, or vertical connection point to extend the drain? Could you see the trap you were connecting to? Would you recognize it if you removed the trap with the tub? You should be able to feel the trap with an electricians fish-tape. If not, you have a problem.

quote; You should be able to feel the trap with an electricians fish-tape.

He offset the drain riser so it might be difficult to get around the elbows and feel a trap. From the description, if it is correct, he should not be having that problem, unless the long pipe from the drain to the trap has gotten fouled and is emitting the odor.

I didn't remove a trap when I pulled the tub. I just added to the inlet. It was horizontal. I never did see the trap but I was told it was probably in the slab or deeper in the dirt. I never had a odor prior to this. I read on here about toilets sometimes siphoning a tub trap. How can I test that? I don't think the extension pipe is fouled, it has only been used for a month. I guess I also need to see if the vent is clogged?

if the pipe you attached to is horizontal, i'm gonna guess there is no trap.
it would be a VERY rare circumstance that the original installing plumber would offset the inlet using 90 deg fittings.

quote; I just added to the inlet. It was horizontal.

The connection to the tub should have been vertical.

so how can I make sure there is or is not a trap? If there is not one, do I have any other options besides tearing out the shower floor and redoing it?

i don't know if this is conclusive or not but i would take a short piece of hose, tuck it down the drain a few inches, wrap a rag around it to sort of seal the drain opening and blow gently into the hose.
if you meet no resistance you have no trap.
if you meet some resistance and you blow harder you should blow some of the trap water out of the trap and then feel no resistance.
as i said, not very scientific but it may work??