Well, just tested that and remembered that I had the same problem once, even in the Opportunities module you wont have the sign there, only the currency next to the field name, the way you should do it for a custom field is like this:
1- Edit the file custom/modules/Accounts/metadata/detailviewdefs.php, search for your custom field (in my case is amount_c) and replace the line that is commented out:
array (
0 =>
array (
'name' => 'amount_c',
'label' => '{$MOD.LBL_AMOUNT} ({$CURRENCY})',
//'label' => 'LBL_AMOUNT',
),
),
2- For the above code to work you need to copy the file modules/Accounts/views/view.detail.php to custom/modules/Accounts/views/view.detail.php in that file add the next code right bellow function display(){
$currency = new Currency();
if (isset($this->bean->currency_id) && !empty($this->bean->currency_id))
{
$currency->retrieve($this->bean->currency_id);
if ($currency->deleted != 1)
{
$this->ss->assign('CURRENCY', $currency->iso4217 . ' ' . $currency->symbol);
}
else
{
$this->ss->assign('CURRENCY', $currency->getDefaultISO4217() . ' ' . $currency->getDefaultCurrencySymbol());
}
}
else
{
$this->ss->assign('CURRENCY', $currency->getDefaultISO4217() . ' ' . $currency->getDefaultCurrencySymbol());
}
3- Make a quick build and repair
The result will be like this
if you really want to add just the symbol on the left side of the amount, you can create a logic_hook the concatenates the amount with the symbol, let me know if you want this guide too.
best regards