They don't store the online data back very far, which is extremely annoying. You can see it between two dates, but only, basically, if those dates are since your last statement! This is something I have complained to them about, but only got a generic "thanks for your comments" response.
This is one of my biggest bug-bears with HSBC. If my NZ bank account can give me data back THREE YEARS why can't HSBC at least give me a couple of months (which seems to be the industry standard).
I had this happen to me on my Barclays UK account a few years ago. I happened to be doing a few transactions in the early hours and i noticed that several thousand pounds was suddenly not showing up via the online banking page. I telephoned the 24hr helpline and was told by a very nice lady ( clearly not expecting anyone to be calling at about 2am..ha ha) that it is quite normal. Sometimes they make a transaction and then remove it in order to 'test' accounts for security/validity or something like that. She also explained that they usually do it during the night as they don't expect people to be accessing their accounts during that time. Hence, normally you never notice it happening.
Having said that, i would agree that if you see money appearing and disappearing on your account frequently, it does look suspicious, the bank should have a fraud department who can investigate the who,how,where and why.
I am not disputing that you were told this explanation but I find this incredulous if this is true. I cannot think of any rational explanation for such an unusual situation. I am far from an IT person but I know from being a former exec in a very large Canadian insurance company that no testing of the live system was done. It was done in a test system entirely.
Like KIA suggests, go to the nearest branch for an explanation but if this is true, that they do this to live accounts, I am in complete shock!!!!