If you are growing on a small scale the number one cause of having the police show up is because someone you know got busted, the police use the tactic of scaring that innocent small time pot smoker into giving up some names for an apparent reduced sentence or fine (which they cant actually authorize anyway as it is up to a judge to determine your punishment, not in fact the police, unless they are going to just let you off with something like deferral to a drug seminar and a fine, which they will choose to do if you help them or not based on the amount of drugs and type).
Its often if they arrest people late at night and they don't ask for a lawyer, or they say its to late to get a lawyer then proceed to try to put pressure on that person to give up their source.
So basically, don't tell ANYONE your growing, if you post on forums do it from an IP address that cant be traced back to you, never use your real name, don't give out any information that could identify you, don't write anything down.
The part about the police watching hydro stores really seems a little far fetched and would depend more on location than anything else. Buying items from a hydroponics shop does not warrant any from of police investigation at all unless they ran the plates of the car you were driving and a lot of known drug arrests or criminal behavior came up. I have friends who have bought small clone tents, lighting systems, fans etc and they are growing herbs, buying things from a shop does not equate to any form of legal investigation, anyone who says there are enough police to follow every person who buys items from a shop has been smoking to much.
What I would say is a combination of suspicious activity would be the most likely cause for investigation, the simple fact is most people who get the police knocking at their doors are stupid enough to bring it on themselves. I have friends who got busted for dealing and of course they were growing in their homes so when the police got a warrant look what other charges they can add. On the other side I know some people who have grown for 10+ years in many different houses never sold any and never been close to getting caught, they grow private they smoke private, they aren't high at work or smoking in their cars (its all about not drawing unnecessary attention).