SQUIRRELLING away one's life savings under the mattress is a horrible risk, as 37-year-old Pang Ko-fai can sadly testify. Mr Pang had kept his secret stash of $100,000 in the confines of his caged bed-space in a Yau Ma Tei hostel - until yesterday when both the cash and the man on the bunk above him went missing. According to police Mr Pang had foolishly told his 35-year-old bed-mate Wong Kam-wah about the money during a night-time chat in their 12th floor Jordan Road hostel. ''After he told his friend about the money he went out at about 11.30 pm on Tuesday and did not come back until 9.30 yesterday morning. ''When he got back he found his money was gone - and so was his friend,'' a police spokesman said. To add insult to injury, the thief had by-passed the padlocked cage around his bed-space by cutting a hole in the bed base of the upper bunk. ''I don't think it is unusual that Mr Pang had so much cash as many Hong Kong people, even those who live in poor conditions, keep large amounts of money hidden rather than using banks,'' the policeman said. ''However, people should realise it is not safe to leave thousands unlocked - especially after telling people about it and then leaving for the night.'' In the dimly lit narrow corridor, flanked on either side by two berth cages, the talk last night centred on the need for increased security. And was Mr Pang at home? ''No, he has gone out . . . probably to look for Wong,'' said one cageman, shaking his head. ''I wish I had that much money. And if I did I wouldn't be telling anyone, especially someone who shares my bunk.''