I set up a Truffle sample project, which contains the following contract:

    import "ConvertLib.sol";

    contract MetaCoin {
      mapping (address => uint) balances;

        event Transfer(address indexed _from, address indexed _to, uint256 _value);

        function MetaCoin() {
            balances[tx.origin] = 10000;
        }

        function sendCoin(address receiver, uint amount) returns(bool sufficient) {
            if (balances[msg.sender] < amount) return false;
            balances[msg.sender] -= amount;
            balances[receiver] += amount;
            Transfer(msg.sender, receiver, amount);
            return true;
        }
        function getBalanceInEth(address addr) returns(uint){
            return ConvertLib.convert(getBalance(addr),2);
        }
        function getBalance(address addr) returns(uint) {
            return balances[addr];
        }
    }

The two last functions only return a value and don't alter the state of the network. Why was there no constant used for these two functions? From what I can tell looking at the logs, in their current form, both functions need mining and cost gas when being called.

I've set up a Truffle sample project, which contains the following contract with the last method added for demonstration purposes (see logs below):

pragma solidity ^0.4.4;

import "./ConvertLib.sol";

// This is just a simple example of a coin-like contract.
// It is not standards compatible and cannot be expected to talk to other
// coin/token contracts. If you want to create a standards-compliant
// token, see: https://github.com/ConsenSys/Tokens. Cheers!

contract MetaCoin {
  mapping (address => uint) balances;

  event Transfer(address indexed _from, address indexed _to, uint256 _value);

  function MetaCoin() {
    balances[tx.origin] = 10000;
  }

  function sendCoin(address receiver, uint amount) returns(bool sufficient) {
    if (balances[msg.sender] < amount) return false;
    balances[msg.sender] -= amount;
    balances[receiver] += amount;
    Transfer(msg.sender, receiver, amount);
    return true;
  }
 
  function getBalanceInEth(address addr) returns(uint){
    return ConvertLib.convert(getBalance(addr),2);
  }
 
  function getBalance(address addr) returns(uint) {
    return balances[addr];
  }

  function getBalanceWithConstant(address addr) constant returns(uint) {
    return balances[addr];
  }
}

The getBalance() function only return a value and doesn't alter the state of the network. Why was there no constant used for this function in the Truffle sample code? From what I can tell looking at the logs, in its current form, it needs mining and costs gas when being called.

Update

Here's the log.

First I call the function with the constant, which just returns the value:

Home:truffle migu$ truffle console
truffle(development)> var meta;
undefined
truffle(development)> MetaCoin.deployed().then(function(instance) { meta = instance;})
undefined
truffle(development)> address = '0xf27c14a83851210c6e26502433fd8193733ace90'
'0xf27c14a83851210c6e26502433fd8193733ace90'
truffle(development)> var tx = meta.getBalanceWithConstant(address);
undefined
truffle(development)> tx
{ [String: '10000'] s: 1, e: 4, c: [ 10000 ] }

Also, the testrpc logs don't show any mining:

eth_call

Then I call the function without a constant:

truffle(development)> var tx1 = meta.getBalance(address);
undefined
truffle(development)> tx1
{ tx: '0xc671857f5fdfb1e7f487cebd32d74a1dfd99a3d54d8201f4435e4595753400be',
  receipt: 
   { transactionHash: '0xc671857f5fdfb1e7f487cebd32d74a1dfd99a3d54d8201f4435e4595753400be',
     transactionIndex: 0,
     blockHash: '0xe12fce4f8e2b3dd8fc0ca840ab5a63e60a8c94e22ab6c5b157fc654ddf4c2f48',
     blockNumber: 6,
     gasUsed: 23146,
     cumulativeGasUsed: 23146,
     contractAddress: null,
     logs: [] },
  logs: [] }

And the testrpc logs show mining activities:

eth_sendTransaction

  Transaction: 0xc671857f5fdfb1e7f487cebd32d74a1dfd99a3d54d8201f4435e4595753400be
  Gas usage: 0x5a6a
  Block Number: 0x06
  Block Time: Sun Mar 12 2017 08:07:27 GMT+1100 (AEDT)

eth_getTransactionReceipt