Are you worried about commercial exploitation or governmental knowledge?
This is a good start:
But you should assume the government receives notification of everything. Hotel Check-ins, pre-paid SIM purchases, etc. Assume little or no personal rights exist in Singapore when it comes to protecting your privacy from the government.
Paranoid much? The Government does not receive notification of "everything". Do you have any idea how much it would cost in terms of computer power alone to track the transactions of 5 million people over the course of just one day? Never mind about having the infrastructure to connect all these transaction sites to a location that can process these transactions, then having someone process it into useable information and someone else to decide if that information can/should be acted upon.
Can they have access to such information? Yes. But most of it is re-active. e.g. You commit a crime, in order to track you down, they search your phone records, bank transactions, get details of your hotel 81 stay where you were accused of murdering some hooker, etc. Even then, this information is only made available with a warrant from the courts. A police officer can't just go up to a bank and request for confidential information and expect to get anything. Most of the time they're not interested in you anyways, unless you've done something to interest them. If you're an especially vocal in your disagreement with them or pose some sort of security risk to the country, they may just keep tabs on you...
Back to the Martin's question, we need more details... Who are you worried will gain access to this information and what level of information do you expect them to access regarding your transactions? I think I already answered the question on the government. Are you asking about a spouse who's suspicious of something and going hotel to hotel asking if you spent the past week at the hotel, then the answer is most likely, No. They aren't going to release information about their guests so easily.
Most reputable hotels or businesses for that matter try to protect their customer's information. I makes plenty of business sense to do so. They know their clients treasure privacy and who's to stay this random person asking about their customers isn't a competitor trying to poach customers?
Other corporations stealing your contact details, possible. Given that most admin staff are underpaid and over worked and they have access to contact info, they may just steal client info and sell it to the highest bidder for a quick buck. Not difficult to arrange.