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Prince Norodom Sihanouk has quietly slipped into Kampuchea (Cambodia) . . . . . . for a secret meeting with his anti-Vietnamese coalition partners. The coalition government cabinet is reported to have met Thursday ''somewhere inside Cambodia,'' though apparently without Son Sann, head of the moderate Democratic faction, who was just returning from Europe.

Enroute to Kampuchea through Bangkok, the former head of state, ousted in a 1970 military coup, met with the Thai foreign minister and ambassadors of the four other members of the Association of Southeast Asian Nations. It is not yet known whether the ASEAN countries offered the prince aid; hints that they might do so have been circulating for the last few weeks.

The prime movers behind the visit seem to be the Chinese, who were apparently worried that the prince might either pull out of the coalition or start negotiations with the Vietnamese. The prince may well have had his most important talks during a three-day stopover in Peking before arriving in Bangkok.

The Chinese went out of their way to treat the prince with honors befitting a head of state. Diplomats believe the Chinese offered him materiel and perhaps military aid.m